Since the myelosuppressed patient with cancer frequently develops pneumonias which are difficult to diagnose, aggressive diagnostic techniques are warranted. This study established a step-wise, orderly method of diagnosing unexplained plumonary infiltrates utilizing transtracheal bronchial brushing, fibroptic bronchoscopy, or percutaneous lung biopsies when other methods have failed. Fifty-seven patients with diffuse or nodular infiltrates have undergone such procedures. Fourteen patients were not clinically infected; 8 proved to have pulmonary metastases; 5 proved to have interstitial fibrosis; and one proved to have a collapsed lobe. Forty-three patients were clinically presumed infected. Twenty-eight were proven to have pulmonary infection. Twenty-four of the 28 were etiologically documented by these procedures. Fifteen of the 43 patients had no etiology established by any method, although 12 of the 15 patients were receiving broad spectrum antibiotics at the time of the procedure. The diagnoses included a variety of microorganisms: bacterial, protozoal, and fungal. Significant but non-fatal complications occurred and were acceptable risks for the patient population. There was no mortality associated with the procedures, and despite pancytopenia, bleeding was a problem in only 3 patients.